1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus, obtaining an image for a subject on the basis of a magnetic resonance signal radiated from the subject in a magnetic field, and a magnetic resonance imaging method.
2. Description of the Related Art
In a magnetic resonance imaging (MRI), a spatial linearity of a gradient magnetic field is a very important factor to allow a spatial position relationship of a subject to be realized in an image. In a general electromagnetic solution, a model having an unlimited length may be supposed. However, in a spatial range of a static magnetic field, a gradient magnetic field, and an RF excitation magnetic field in actual, the length is limited due to an effective field of view for imaging a human body. For this reason, a linearity of the gradient magnetic field or a spatial uniformity of the static magnetic field or the RF excitation magnetic field deteriorates at a peripheral portion of each magnetic field irrespective of a center portion of each magnetic field.
As described above, there is a certain limitation in the spatial linearity of the gradient magnetic field or the spatial uniformity of the static magnetic field and the RF excitation magnetic field as long as the imaging is carried out within the limited length. In the related art, the efficient field of view (efficient FOV) is expressed in combination of the magnetic fields. Then, in terms of a manual or an operator instruction, it is emphasized that it is desirable to carry out the imaging within about 50 cm or to carry out the imaging at a position closest to a center of a magnet to obtain an image having an excellent image quality.
For this reason, in general, an operator who wants to reliably obtain a satisfactory image sets a small imaging region so that an imaging region is reliably included in the efficient FOV. That is, since the imaging uses only a part of a region capable of carrying out the satisfactory imaging in actual, efficiency is poor. In the case where the operator sets the large imaging region by predicting that the satisfactory imaging is carried out in a region larger than the efficient FOV, the imaging region may not be included in the region capable of carrying out the satisfactory imaging in actual. As a result, in this case, the image quality deteriorates.
In recent years, in order to reduce a subject' psychological pressure, a decrease in size is realized in a direction of a gantry bed. Thus, a spatial uniformity of the static magnetic field and the RF excitation magnetic field deteriorates. Additionally, a high SR (slew rate) is required in a sequence of an EPI (echo planar imaging) or an SSFP (steady state free precession) system. Additionally, it is required to reduce a magnetic stimulation derived from dB/dt (magnetic-field-strength time variation rate) generated by a switching of the gradient magnetic field. In order to satisfy both requirements, a gradient magnetic field coil is often designed in a nonlinear shape, and a spatial linearity of the gradient magnetic field tends to gradually deteriorate. Further, the magnetic stimulation indicates that current in accordance with a variation in magnetic flux at an initial rise of the gradient magnetic field generated by the switching of the gradient magnetic field is formed in a pulse shape and flows to a nerve to occur paralysis.
PATENT DOCUMENT 1: Japanese Patent Application Laid-Open No. 5-137707